Chair back



FIG. 1 is a perspective view of a chair back showing my new design;

FIG. 2 is a plan view of thereof;

FIG. 3 is a rear elevation view thereof;

FIG. 4 is a bottom view thereof;

FIG. 5 is a front view thereof; and,

FIG. 6 is a left side elevation view thereof, the right side elevation being a mirror image thereof.

The broken lines in the Figures are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a chair back, as shown and described. 